Europium-activated barium magnesium aluminate (BAM) phosphors are widely used as the blue-emitting component of the phosphor blends in most fluorescent lamps intended for white light generation. BAM phosphors also serve as the blue-emitting pixels in plasma display panels (PDPs). Despite its wide use, BAM is notorious for its shortcomings in brightness and maintenance, particularly in those applications involving exposure to high ultraviolet (UV) and vacuum ultraviolet (VUV) fluxes. Because of these shortcomings, the blue BAM emission is reduced at a significantly faster rate over time than the emissions of the other color components in the blends or pixels. This results in a loss of lumens and a color shift in the overall light output.
Theoretical and experimental investigations of various BAM compositions over the past few years have yielded clues about the degradation mechanisms involved in the phosphor's maintenance. A prolonged exposure to radiation with photons having energies above 5 eV (wavelengths less than 254 nm) causes a reduction in the phosphor's brightness and changes in the spectral power distribution of the phosphor's emission. These effects can be observed by spectroscopic methods after hundreds of hours of lamp operation or by a short period of high-intensity laser irradiation (e.g., a 193 nm excimer laser). In addition to an approximate 25% decrease in brightness after 500 hours of operation, there is an increase in the long wavelength side of the emission band of the phosphor. Very likely, these effects are linked to electron and hole centers formed during the phosphor synthesis and/or later generated as a result of ion bombardment and UV/VUV irradiation during lamp operation. In particular, electron centers (oxygen vacancies that have captured zero, one or two electrons) are believed to compete with the europium activator ions for UV/VUV photons and may also absorb a portion of the visible light emissions from the phosphor. It is also possible that oxygen vacancies with zero or one electron may capture electrons produced, for example, from the photoionization of Eu2+ to Eu3+ upon 185 nm UV irradiation. If the number of defects capable of capturing electrons from the ionization of the europium activator ions is comparable to the number of europium ions in the lattice, or becomes so during the operating life of the phosphor, a serious reduction of the emission intensity will follow over time.